Lithium-ion batteries play a vital role in the development of many energy-dependent applications, such as electric vehicles, portable electronics, and renewable energy storage. However, with the wide adoption of lithium-ion batteries over the last two decades, technology limitations that impede more widespread implementation of lithium-ion batteries have become evident. Foremost concerns deal with insufficient energy storage, poor safety, high cost, and inadequate lifetime of lithium-ion batteries, with ancillary issues including poor low temperature performance and problematic recyclability.
Electrolyte solutions used in lithium-ion batteries are known to be unstable at high temperatures and high voltages. Over time, the electrolyte solution turns into a tar-like material at high temperatures, which has precluded the widespread implementation of lithium-ion batteries in vehicular applications, such as in hybrid electric vehicles (HEVs) and plug-in type hybrids (PHEVs). The electrolyte solutions include carbonate-based solvents, such as dimethyl carbonate (DMC), ethylene carbonate (EC), or ethylmethyl carbonate (EMC). However, the carbonate-based solvents are problematic due to their high volatility, flammability, and decomposition at even modestly elevated temperatures, such as low as 60° C.
To reduce the flammability of the electrolyte solution, organophosphorus compounds, such as phosphates and cyclic phosphazenes, have been investigated as an additive or co-solvent to the electrolyte solution. Phosphoranimine compounds are known in the art as synthetic intermediates in the formation of polyphosphazene compounds or cyclic phosphazene compounds. However, beyond being synthetic intermediates, phosphoranimine compounds have thus far seen few uses themselves.
Phosphoranimine compounds have been disclosed for use in positive electrodes of lithium primary cells and for use in electrolyte solutions in combination with an aprotic organic solvent. The phosphoranimine compound includes a phosphorus-containing group, such as a phosphine oxide group, a sulfur-containing group, such as a sulfone group, or a nitrogen-containing group directly bonded to a nitrogen atom of the phosphoranimine compound.
As described in “An Electrochemically Compatible and Flame-Retardant Electrolyte Additive for Safe Lithium Ion Batteries” to Wu et al., J. Power Sources 227 (2013) 106-110, a phosphazenic compound was synthesized and tested as an electrolyte additive. The phosphazenic compound was triethoxyphosphazen-N-phosphoryldiethylester (PNP), which included a phosphine oxide functional group bonded to a nitrogen atom of the phosphazenic compound. The electrochemical characterization of the PNP-containing electrolyte was tested in MCMB/Li half-cells at 0% loading, 10% loading, 20% loading, 50% loading, and pure PNP. The PNP-containing electrolyte showed a 20% decrease in capacity at a 20% loading. At 10% loading, there was little observed difference when compared to traditional electrolytes. However, loadings in excess of 20%, for example, at 50% and 100%, were not able to cycle effectively.
It would be desirable to further improve electrolyte solutions to advance the usefulness of lithium-ion, and other, batteries.